Rett syndrome (RS) is a neurodevelopmental disorder which develops only in females after apparently normal psychomotor development for the first six months of life. The etiology and pathogenesis of RS is unknown. The purpose of this study is to evaluate the effect of L-carnitine on behavioral, motor, respiratory, neurophysiological, and nutritional features of RS. Twenty Stage III RS girls will be randomized to a double-blind, crossover, dose escalating study of L-carnitine vs placebo. Physical and neurological exams, stadiometry, anthropometry, motor function, parental behavior diaries, laboratory, and a polygraphic studies will be done to determine drug efficacy. If L-carnitine proves to be an effective treatment for RS, this could lead to a better understanding of the pathophysiology of this disorder.